Abstract: Brillouin Optical Time-Domain Reflectometer (BOTDR) is a newly developed distributed measuring technique for strains. In this paper, the merits and the basic measurement principle of BOTDR is presented. The feasibility and advantages of the distributed strain monitoring of structures in geotechnical engineering are demonstrated by a case study for strain monitoring in a tunnel. At the end, several key issues for applications of BOTDR are discussed, including enhancement of spatial resolution, layout of the sensing optical fiber and the health monitoring and damage diagnosis.